Thistles are invasive weeds that grow deep roots, which makes them difficult to control. Even after you pull or dig them out repeatedly, they can continue to grow back. Because of this, using an herbicide that penetrates the foliage of the thistle and works its way down to the roots to kill it is one of the best methods to eradicate thistles once and for all. However, there are a few things to consider to ensure you're picking the right herbicide for the job.
Verify what kind of thistle you have. You can take a sample to your local county extension office or to your local nursery, if necessary. Two common thistles are bull and Canada thistles. Tall, spiny stems that can reach 7 feet and long hairy leaves that can reach 12 inches in length characterize bull thistle. Canada thistle has leaves that are hairless near the top but that may be or not be hairless on the lower end of the leaves. It, too, grows tall---up to about 6 feet.
Observe where your thistle is growing. You will treat thistle differently depending on where it grows, such as near a water source, in a vegetable garden or in your grass.
Determine at what stage of growth your thistle is. Spring and fall are typically when thistle is actively growing. Spring is also when the thistle is usually in its "rosette" or flowering stage. Early spring is typically the emergence and prebud stage. In the summer, the thistle may be growing but not at its greatest speed. In the winter, thistle is usually dormant.
Find out what herbicides are illegal to use in your state. Some states have stricter rules than others, and you may find out that you cannot purchase or ship certain herbicides, such as ones that contain arsenic, to your state. Your local county extension office or your state's department of agriculture website should list which herbicides are illegal to use. This information is also posted on the herbicide's label.
Read the herbicide's label carefully and select one based on the preceding information. Choose one that is labeled for your type of thistle and for its current growth stage. Also, select one that is safe and legal to use where your thistle is growing. For example, an herbicide with the ingredient chlorsulfuron (sold as Telar) is a suitable herbicide to use to eradicate Canada thistle that is growing in your lawn in the early spring when the shoots begin to rapidly emerge until it begins to grow buds. However, since you cannot legally use chlorsulfuron in some states, including California and Montana, select another herbicide instead, such as one that contains aminopyralid.